If any blood contain antigen A but no antigen B, the blood group will be A blood group. If any blood contains antigen B but no antigen A, the blood group will be B blood group. If any blood contains both antigen A and antigen B, the blood group will be AB blood group. If any blood contains none of them (antigen A and antigen B ) will be O blood group.
Blood group of offspring depends on parent’s blood group. If father and mother are both bearing O blood group, their offspring’s blood group must be O. If one of them bearing A and other is bearing B blood group, offspring’s blood group may be A, B, AB or O. I am trying to describe the whole matter genetically but avoiding scientific terms as possible as I can.
Every child gets their traits (genetically transmitted traits, not all traits) from their parents. They get 50% of their traits from their father and 50% from their mother. In other word, father transmits his 50% and mother transmits her 50% to their child. Every trait is controlled by a pair of genes.
Let A blood group is controlled by “A_” gene pair,
B blood group is controlled by “B_” gene pair,
AB blood group is controlled by “AB” gene pair,
O blood group is controlled by “_ _” gene pair,
Now you can easily calculate the offspring’s possible blood group(s) if you know the parent blood group. For example, if one of them have A blood group (A_) and other have O blood group (_ _). A blood group can be divided as “A” and “_” and “O” blood group can be divided by “_” and “_” . So the possible combination of A, _ and _, _ are A _ and _ _ ie., blood group A, and O. So, if parents bearing blood group O and A, offspring blood group will be A or O not B or AB.
Another example to make this more clear, let parents blood groups are B and O. B blood group contains “B” and “_” on the other hand O blood group contains only “_” (_ _). So, the possible combination of B, _ and _, _ are “B_” and “_ _” ie., blood group B and blood group O. So the parents bearing blood group B and O will result offspring only B and O blood groups.
In this process we can calculate the possible blood group of offspring if we know the blood group of parents.

